Canola is comprised of three major species that are modified forms (using traditional plant breeding methods) of rapeseed or brown mustard: Brassica napus (also known as Argentine canola), Brassica rapa (also known as Polish canola), or Brassica juncea (canola quality brown mustard). B. napus, with its 19 chromosomes, originated from a cross between B. oleracea (e.g., cabbage, 9 chromosomes) and B. rapa (e.g., turnip, 10 chromosomes). The same is true for B. juncea, which originated from a cross between B. nigra (e.g., black mustard) and B. rapa (e.g., turnip). In the past few years, another mustard species (Sinapis alba) has been modified to produce a similar oil profile and meal quality as canola oil. Canola has become increasingly more important to the world, through breeding for better oil quality and improved processing techniques.
Clubroot is a serious soil-borne disease of cruciferous crops. Clubroot is caused by the pathogen Plasmodiophora brassicae (P. brassicae), which is a protist and an obligate parasite. In canola, it causes swellings or galls to form on the roots and restricts the flow of water and nutrients from roots to aboveground plant tissues. Clubroot also causes stunting through reduced growth, and wilting of leaves is observed under water stress. The disease ultimately causes premature death of the plant. Yield losses due to clubroot are about half of the percentage of infected stems. Severe field infestations by clubroot can cause total yield loss.
Different pathotypes of P. brassicae appear to dominate in different canola growing regions. For example, in Canada, pathotypes 3 and 5 were observed in populations from the Alberta region, whereas pathotype 6 was found in populations from British Columbia and Ontario (Strelkov, Can. J. Plant Pathol. 28:467-74(2006)). Hildebrand and Delbridge (1995) characterized 10 populations of P. brassicae collected from various cruciferous crops in Nova Scotia, and identified eight as pathotype 3, and one each as pathotypes 1 and 2. Similarly, pathotypes 1 and 2 are most prevalent in France while pathotypes 3 and 5 are also found in selected French regions.
Pathotype 5x is a recently discovered pathotype for clubroot. Pathotype 5x is so named because the resistance profile resulting from the usual screening panel of germplasm suggests that it is pathotype 5, but lines resistant to 5 are not resistant to this pathotype. Currently, the origin of pathotype 5x is unclear. It could have originated from genetic breakdown of a previously existing pathotype or could have been a pre-existing, but rare, pathotype. Pathotype 5x has been found to infect all current tolerant canola varieties in Canada. It is predicted that additional novel P. brassicae pathotypes or pathotype variants may emerge or be uncovered after more extensive field monitoring and resistance profiling. Strategies for combating clubroot infections by new P. brassicae pathotypes are in need.
Currently, there are no economical control measures that can remove the clubroot disease from a canola field once it has been infested. However, it is possible to curtail the spread and reduce the incidence and severity of infection. A number of strategies have been recommended for managing clubroot, including liming of the soil, application of fungicides, use of resistant cultivars, and crop rotation. However, not all of these methods may be practical or affordable. For instance, large amounts of lime may be necessary to increase soil pH sufficiently to reduce disease severity, making this strategy impractical in field crops. Similarly, control of clubroot through the use of fungicides is not always consistent, and may be prohibitively expensive. Crop rotation away from susceptible crops is an effective management strategy, but rotation breaks must be long, as resting spores of the pathogen can survive in the soil for extended periods of time. The use of genetically resistant cultivars is one of the most economically and environmentally desirable strategies for clubroot control.
Several sources of resistance to clubroot have been described within the Brassica genus. Some resistances are monogenic, some polygenic, some are dominant, some recessive. Monogenic dominant resistances have been described in B. rapa and B. napus, such as for example a monogenic dominant resistance in the B. rapa Chinese cabbage (Yoshikawa (1983) Japan Agricultural Research Quarterly, Vol. 17, no. 1, p. 6-11). Using a resynthesized B. napus line (from B. oleracea ‘Bohmerwaldkohl’× B. rapa ECD-04), Werner et al. reported nineteen QTL on chromosomes N02, N03, N08, N13, N15, N16 and N19 giving resistance to seven different clubroot isolates. Theor Appl Genet, 116:363-72 (2008). These QTLs were designated as PbBn-Korp-1, PbBn-Korp-2, PbBn-Korp-3, PbBn-Korp-4, PbBn-Korp-5, PbBn-k-1, PbBn-k-2, PbBn-k-3, PbBn-01.07-1, PbBn-01.07-2, PbBn-01.07-3, PbBn-1-1, PbBn-1-2, PbBn-01:60-1, PbBn-01:60-2, PbBn-01:60-3, PbBn-01:60-4, PbBn-e4x04-1, and PbBn-a-1. See Werner et al. Theor Appl Genet, 116:363-72, at 369, Table 2 (2008).
Mendel and Tosca, two winter canola varieties, were first introduced to the European seed market in 2000. They both originated in a resynthesized B. Napus form and were reported to possess clubroot resistance to specific pathotypes. See Diedrichsen et al., J Plant Growth Regul, 28:265-81 (2009). Despite many studies related to the clubroot resistance in Mendel, relatively little is known about the genetic basis of the clubroot resistance in Tosca. Further, transfer of clubroot resistance from winter canola to spring canola could be challenging due to high levels of potential genetic drags associated with the transfer. See Hawkins et al., Planta 216:220-26 (2002).
There is a need in canola breeding to identify canola germplasm providing resistance to newly emerging pathotypes (e.g., pathotype 5x) and to develop elite canola varieties that can be grown in regions infected with these new pathotypes. There is also a need to identify resistance loci, haplotypes, and chromosomal intervals that confer or are linked to clubroot resistance, e.g., resistance to pathotype 5x. Additionally, there is a need for a rapid, cost-efficient method to assay, monitor, and introgress clubroot resistance alleles in canola.